1. Field of the Invention
The present invention relates to an image processing device for recognizing the outline of a moving target such as human and vehicle from a plurality of still images.
2. Disclosure of the Prior Art
In the past, an image processing device using a background subtraction method or a frame subtraction method has been proposed to recognize a moving target such as human and vehicle. In these methods, image processing is performed to grayscale images, and outline images are used to minimize the influence of a change in brightness such as lighting.
The principle of the image processing device according to the background subtraction method is explained in brief referring to FIG. 11. In this image processing, a background image (α) including no moving target is previously taken, and stored. Then, a plurality of images (β) each including the moving target are taken in time series. Each of the obtained images is converted into a binary outline image by use of a SOBEL operator, and stored. By determining a difference between a required outline image and the background image, a region having a predetermined amount of change therebetween is recognized as the region corresponding to the moving target (X).
However, the step of previously preparing the background image is a messy step. In addition, in a case of using an average of the images taken within a required time period as the background image, when the moving target is in a state of rest within the time period, a recognition error of the moving target may occur from the subsequent image processing. This becomes a cause of decreasing the reliability of the image processing device.
As another image processing device using the background subtraction method, Japanese Patent [Early] Publication No. 6-337938 discloses an image processor and a distance measuring device. In this image processing, a first image that is a background image including no moving target, a second image including the background and a moving target with a first speed or less, and a third image including the background and another moving target with a second speed or less are used. A difference between the first and second images is determined to obtain a first subtraction image. Similarly, a difference between the second and third images is determined to obtain a second subtraction image. In addition, a difference between the third image and an image provided by an image input means is determined to obtain a third subtraction image. According to the first to third subtraction images, motions of the moving targets with different speeds can be analyzed at a video rate.
Since this image processor uses the background subtraction method, the messy step of previously preparing the background image is still needed. In addition, since it is difficult to precisely generate the backgrounds of the first to third images, there is a fear of deteriorating the accuracy of analyzing the motions of the moving targets with different speeds. Moreover, there is another problem that this image processing is sensitive to a change in brightness.
On the other hand, the principle of the image processing device according to the frame subtraction method is explained in brief referring to FIG. 12. In this image processing, a subtraction |(T)−(T−ΔT1)| between first and second images taken at different times (T, T−ΔT1) is calculated, so that a region with a predetermined amount of change therebetween is recognized as the region corresponding to the moving target.
According to this method, the step of previously preparing the background image is no longer necessary. However, in the case of determining the absolute-value subtraction, the outline of the moving target (X) may be extracted such that two outlines partially overlap with each other, as shown in FIG. 12. On the other hand, in the case of determining a saturated subtraction, the problem of the absolute-value subtraction can be solved. However, when is a negative value is obtained by the subtraction, it is regarded as zero. Therefore, when an outline of the background hidden by the target appears in the image by a movement of the target, there is a problem that the outline of the background (Y) is accidentally extracted together with the outline of the moving target (X), as shown in FIG. 13.
As another image processing device using the frame subtraction method, Japanese Patent [Early] Publication No. 2000-82145 discloses an object extraction device of recognizing the moving target from three images taken in times series. In this image processing, a set of first, second and third images each including the moving target are taken at different times (T−ΔT1, T. T+ΔT1). From the frame subtractions between the first and second images and between the second and third images, a pair of subtraction images are obtained. In each of the subtraction images, a background region is determined, so that the remaining region is selected as a candidate region of the moving target. Finally, by determining an intersection of the candidate regions, the outline of the moving target at the time (T) can be extracted.
However, when the moving target is parts of the human body such as arms and legs, there is a case that the parts with motions at the first time are different from them with motions at the second time, and similarly the parts with motions at the second time are different from them with motions at the third time. In such a case, the outline extracted by the frame subtraction between the first and second images becomes different from the outline extracted by the frame subtraction between the second and third images. As a result, the outline obtained by determining the intersection of the candidate regions may lose a considerable part of the outline of the moving target to be extracted.
Thus, from the viewpoint of further improving the accuracy of recognizing the moving target without including the background information, there is still room for improvement in conventional image processing devices.